In recent years, rechargeable electric storage devices such as battery cells (such as lithium ion battery cells and nickel hydrogen battery cells) and capacitors (such as electric double layer capacitors) have been employed as a power source, for example, for vehicles (such as automobiles and motorcycles) and various equipment (such as mobile terminals and laptop computers).
Such an electric storage device generally includes external terminals to which conductors such as cables and bus bars are connected, an electrode assembly in which electrode plates are multilayered, and current collecting members that electrically connect the electrode assembly to the external terminals.
The electrode assembly includes a positive electrode plate and a negative electrode plate. The positive electrode plate and the negative electrode plate are layered together while they are insulated from each other. The positive electrode plate and the negative electrode plate are arranged to be displaced from each other in the width direction. This allows a projection in which only the positive electrode plate is layered to be formed at one end in the width direction of the electrode assembly and a projection in which only the negative electrode plate is layered to be formed at the other end in the width direction of the electrode assembly.
The electric storage device includes a positive current collecting member bonded to the positive electrode plate (projection) and a negative current collecting member bonded to the negative electrode plate (projection). These current collecting members have a common basic configuration. Each of the current collecting members has a connector welded to one of the electrode plates (the positive electrode plate or the negative electrode plate).
A specific description is given below. The current collecting member includes a current collector that is arranged along the electrode plate (the positive electrode plate or the negative electrode plate) of the electrode assembly and is electrically connected to the external terminal. Such a current collecting member of this type includes a backing member or a clip member in addition to the current collector. The backing member is arranged along the electrode plate (the positive electrode plate or the negative electrode plate) of the electrode assembly. The clip member has a pair of facing pieces facing each other with the electrode plate (the positive electrode plate or the negative electrode plate) of the electrode assembly interposed therebetween. The current collector is stacked on one of the pair of facing pieces.
In a current collecting member including the backing member, the portion of the current collector that extends along the electrode plate and the backing member are bonded (welded) to the electrode plate as a connector so as to be conductible. Further, in a current collecting member including the clip member, the portion of the current collector that extends along the electrode plate and the pair of facing pieces are bonded (welded) to the electrode plate as a connector so as to be conductible.
In such an electric storage device of this type, the electrode assembly and the current collecting member are welded together by vibration welding. Vibration welding is a welding method in which, while welding objects are stacked together, vibration is applied to the objects, so that the two objects are fused (welded). Ultrasonic vibration welding is widely adopted as one of vibration welding. Such ultrasonic vibration welding uses a horn to which ultrasonic vibration is applied, and an anvil that sandwiches the welding objects with the horn. In the electric storage device having the aforementioned configuration, while the electrode plate (projection) of the electrode assembly and the connector of the current collecting member are sandwiched by the horn and the anvil, ultrasonic vibration is applied to the horn. Thus, the electrode assembly (electrode plate) and the current collecting member (connector) are welded together.
In vibration welding, two welding objects are fused by application of vibration to the welding objects so as to generate friction between the welding objects stacked together. Therefore, the welding objects do not appropriately fuse together, if vibration energy does not efficiently propagate to the welding objects. Accordingly, in the production process of the electric storage device, there are cases where the electrode assembly (electrode plate) and the current collecting member (connector) do not sufficiently fuse (penetrate) together even if they are subjected to vibration welding. As a result, in the electric storage device of this type, there is a concern that a bonding failure between the electrode assembly and the current collecting member occurs. Further, it is another concern that such a bonding failure causes damage to the electrode plate of the electrode assembly.